1. Field of Invention
This invention is directed to cavity resonators and optical devices involving cavity resonators as a component.
2. Description of Related Art
Cavity resonators are components in optical devices such as lasers, filters, modulators, converters and light-emitting diodes. An important class of cavity resonators are dielectric cavity resonators, which have been used to make dielectric cavity lasers, filters and other devices. Dielectric cavity lasers operate by providing feedback to an optical gain medium, by total internal reflection at a dielectric interface forming the walls of the resonator bounding the gain medium. Well-known examples are disk and cylinder lasers that emit from whispering gallery modes that circulate around the perimeter of the cavity. Disk and cylinder lasers have been proposed and fabricated in which a laser diode structure is formed with a circular cross-section. The larger the diameter of the circular resonator, the less evanescent leakage there is from whispering gallery modes, which effectively increases the reflectivity of the sidewalls and increases the Q of the resonator. Therefore, large diameter circular disk and cylinder lasers have a relatively large gain×length product, and are capable of exceedingly low threshold currents.
One of the disadvantages of circular dielectric cavity lasers is that light output only occurs through near-field evanescent leakage through the side walls. This occurs because the incidence angle of the light on the walls is above the critical angle for total internal reflection. Therefore, coupling light into or out of the resonator is accomplished primarily by near-field coupling. Consequently, an input or output fiber has to be placed sufficiently close to the resonator that the evanescent fields that link the optical regions of the dielectric cavity laser and the fiber are appreciable. Therefore, positioning the input/output fiber with respect to the resonator has to be carefully controlled and the output power tends to be low.
Another disadvantage of circular dielectric cavity lasers is that the laser emission is isotropic. That is, light is emitted from the circular dielectric cavity laser equally from all circumferential positions, i.e., equally along all 360° of the outer surface of the circular dielectric cavity laser. As a result, the light output from circular dielectric cavity lasers cannot be focused by the usual optics and injected into, for example, a fiber optic cable without large losses. For these reasons, there are few applications for circular dielectric cavity lasers.
A number of alternative dielectric cavity laser designs attempting to localize the output of a dielectric cavity laser into well-defined directions have been proposed. One such alternative design is referred to as an asymmetric resonant cavity laser. The asymmetric resonant cavity laser is a dielectric cavity laser with a cross-section smoothly deformed from circular symmetry. Such lasers can emit from either deformed whispering gallery modes or from librational modes such as the bow-tie mode.
FIG. 1 shows one exemplary embodiment of an asymmetric resonant cavity laser, which emits from the points of maximum curvature 11 and 13 located near the poles 10 and 12 of the cavity. The asymmetric resonant cavity laser shown in FIG. 1 has two degenerate directions in which the whispering gallery modes can circulate, clockwise or counterclockwise. The asymmetric resonant cavity laser can be fed by a waveguide 14 located on the top of the device. As shown in FIG. 1, the waveguide 14 couples an optical signal λ into the asymmetric resonant cavity laser, into the clockwise circulating whispering gallery mode. The asymmetric resonant cavity laser outputs the light 20 along a tangent to the point of maximum curvature 13, into an output fiber 16, at the point of maximum curvature 13. The device may also output some of the generated light back into the input fiber 14, as the asymmetric resonant cavity laser generally outputs at least two emitted beams 18 and 20, each emitted along the tangent lines at each point of maximum curvature 11 and 13.
Another embodiment of an asymmetric resonant cavity optical device is the librational mode semiconductor laser, which emits from a bow-tie mode that does not circulate around the periphery. The boundary of this laser resonator is smooth and the beams are not outcoupled by a local perturbation. This laser has high output power and directional emission but produces four output beams.
Another class of lasers is the vertical cavity surface emitting laser diode (VCSEL). In a VCSEL structure, the optical feedback is provided by a pair of distributed Bragg reflector (DBR) mirror stacks located on top and bottom of the active region. The distributed Bragg reflector mirror stacks form a cavity perpendicular to the semiconductor layer. This vertical resonator provides an output beam normal to the surface of the semiconductor layers. However, there are a number of drawbacks associated with such a VCSEL approach. Because the cavity is oriented in the vertical direction, the effective cavity length is the thickness of the multi-quantum well active region, which is only a few tens of nm. Therefore, highly reflective mirrors (reflectivity greater than 99.9%) are required to increase the effective gain×length product. In some material systems, there may be no feasible way to achieve the required reflectivity. In addition, the output power of VCSELs is comparatively small, because of the small gain×length product. However, VCSELs also provide a number of advantages, such as fabrication in two-dimensional arrays, wafer level device testing, single longitudinal mode operation, and improved coupling to optical fibers.